Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329, incorporated herein by reference in its entirety. The carbonylation catalyst contains rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms. Likewise, because the nature of the halide promoter is not generally critical, a large number of suitable promoters, most of which are organic iodides, may be used. Most typically and usefully, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved.
A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol with carbon monoxide. The catalyst contains rhodium and/or iridium and a halogen promoter, typically methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide through a liquid reaction medium in which the catalyst is dissolved. The reaction medium comprises acetic acid, methyl acetate, water, methyl iodide and the catalyst. Commercial processes for the carbonylation of methanol include those described in U.S. Pat. No. 3,769,329, the entireties of which is incorporated herein by reference. Another conventional methanol carbonylation process includes the Cativa™ process, which is discussed in Jones, J. H. (2002), “The Cativo™ Process for the Manufacture of Acetic Acid,” Platinum Metals Review, 44 (3): 94-105, the entirety of which is incorporated herein by reference.
The AO™ process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in U.S. Pat. Nos. 5,001,259; 5,026,908; and 5,144,068; and EP0161874, the entireties of which are incorporated herein by reference. As disclosed therein, acetic acid is produced from methanol in a reaction medium containing methyl acetate (MeAc), methyl halide, especially methyl iodide (MeI), and rhodium present in a catalytically effective concentration. These patents disclose that catalyst stability and the productivity of the carbonylation reactor can be maintained at high levels, even at very low water concentrations, i.e., 4 weight percent or less, (despite the prior practice of maintaining approximately 14-15 wt. % water) by maintaining in the reaction medium, along with a catalytically effective amount of rhodium, at least a finite concentration of water, e.g., 0.1 wt. %, and a specified concentration of iodide ions over and above the iodide ion that is present as hydrogen iodide. This iodide ion is a simple salt, with lithium iodide being preferred. The salt may be formed in situ, for example, by adding lithium acetate, lithium carbonate, lithium hydroxide or other lithium salts of anions compatible with the reaction medium. The patents teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid, especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, a high degree of catalyst stability and reactor productivity is achieved even when the liquid reaction medium contains water in finite concentrations as low as 0.1 wt. %. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e., resistance to catalyst precipitation, especially during the product recovery steps of the process. In these steps, distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide, which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium.
U.S. Pat. No. 5,144,068, the entirety of which is incorporated herein by reference, discloses a process for producing acetic acid by reacting methanol with carbon monoxide in a liquid reaction medium containing a rhodium (Rh) catalyst and comprising water, acetic acid, methyl iodide, and methyl acetate, wherein catalyst stability is maintained in the reaction by maintaining in said reaction medium during the course of said reaction 0.1 wt. % to less than 14 wt. % of water together with (a) an effective amount in the range of 2 wt. % to 20 wt. % of a catalyst stabilizer selected from the group consisting of iodide salts which are soluble in said reaction medium in effective concentration at reaction temperature, (b) 5 wt. % to 20 wt. % of methyl iodide, and (c) 0.5 wt. % to 30 wt. % of methyl acetate. Suitable iodide salts may be a quaternary iodide salt or an iodide salt of a member of the group consisting of the metals of Group IA and Group HA of the Periodic Table.
Carbonyl impurities, such as acetaldehyde, that are formed during the carbonylation of methanol may react with iodide catalyst promoters to form multi-carbon alkyl iodides, e.g., ethyl iodide, propyl iodide, butyl iodide, pentyl iodide, hexyl iodide, and the like. It is desirable to remove multi-carbon alkyl iodides from the reaction product because even small amounts of these impurities in the acetic acid product tend to poison the catalyst used in the production of vinyl acetate, a product commonly produced from acetic acid.
Conventional techniques to remove such impurities include treating the crude acid product streams with oxidizers, ozone, water, methanol, activated-carbon, amines, and the like. Such treatments may or may not be combined with distillation of the acetic acid. The most typical purification treatment involves a series of distillations of the final product. It is also known to remove carbonyl impurities from organic streams by treating the organic streams with an amine compound such as hydroxylamine, which reacts with the carbonyl compounds to form oximes, followed by distillation to separate the purified organic product from the oxime reaction products. However, the additional treatment of the final product adds cost to the process, and distillation of the treated acetic acid product can result in additional impurities being formed.
While it is possible to obtain acetic acid of relatively high purity, the acetic acid product formed by the low-water carbonylation process and purification treatment described above frequently remains somewhat deficient with respect to the permanganate time due to the presence of small proportions of residual impurities. Because a sufficient permanganate time is an important commercial test, which the acid product may be required to meet to be suitable for many uses, the presence of impurities that decrease permanganate time is objectionable. Moreover, it has not been economically or commercially feasible to remove minute quantities of these impurities from the acetic acid by distillation because some of the impurities have boiling points close to that of the acetic acid product or halogen-containing catalyst promoters, such as methyl iodide. It has thus become important to identify economically viable methods of removing impurities elsewhere in the carbonylation process without contaminating the final product or adding unnecessary costs.
Macroreticulated or macroporous strong acid cationic exchange resin compositions are conventionally utilized to reduce iodide contamination. Suitable exchange resin compositions, e.g., the individual beads thereof, comprise both sites that are functionalized with a metal, e.g., silver, mercury or palladium, and sites that remain in the acid form. Exchange resin compositions that have little or no metal-functionality do not efficiently remove iodides and, as such, are not conventionally used to do so. Typically, metal-functionalized exchange resins are provided in a fixed bed and a stream comprising the crude acetic acid product is passed through the fixed resin bed. In the metal functionalized resin bed, the iodide contaminants contained in the crude acetic acid product are removed from the crude acid product stream.
U.S. Pat. No. 6,657,078 describes a low-water process that uses a metal-functionalized exchange resin to remove iodides. The reference also avoids the use of a heavy ends column, resulting in an energy savings.
The metal-functionalization of exchange resin compositions often involves significant processing and expense, often costing orders of magnitude more than resins that are not metal-functionalized. Often the process steps associated with the functionalization varies very little with regard to the actual amount of metal that is deposited on the exchange resin. For example, the processing necessary to functionalize 50% of the active sites of a quantity of exchange resin is quite similar to the processing necessary to functionalize 10% of the active sites of the same quantity of exchange resin. Because the entire quantity of exchange resin requires processing, however, both the 50%-functionalized exchange resin and the 10%-functionalized resin require significantly more processing than the same quantity of non-functionalized resin.
Other ion exchange resins have been used to remove iodide impurities from acetic acid and/or acetic anhydride. There is disclosed in U.S. Pat. No. 5,220,058 the use of ion exchange resins having metal exchanged thiol functional groups for removing iodide impurities from acetic acid and/or acetic anhydride. Typically, the thiol functionality of the ion exchange resin has been exchanged with silver, palladium, or mercury.
In addition to iodide contaminants, metals from the walls of the vessels used in the acetic acid production system often corrode and dissolve into the crude acetic acid product compositions. Thus, conventional crude acid product streams often comprise corrosion metal contaminants as well as iodide contaminants. These corrosion metals are known to interfere with the carbonylation reaction or accelerate competing reactions such as the water-gas shift reaction. Typically, these corrosion metals may be removed from the process streams by passing the streams through resin beds comprising standard macroreticular or macroporous cationic exchange resins.
In a case where a silver, mercury or palladium exchanged resin is utilized, however, the soluble corrosion metal cations may detrimentally displace the metal-functionalized sites of the exchange resins. As such, these exchange sites are unable to capture/remove the iodide contaminants. The lifetime of the functionalized resin, with regard to iodide removal, is shortened by the presence of corrosion metals. Often a pre-determined portion of the sites of the exchange resin composition are functionalized, thus leaving the remainder of the sites in the acid form As a result, the acid sites capture much of the corrosion metals while many of the functionalized sites remain available for iodide removal. Although this technique may improve the lifetime of exchange resins, the partial functionalization of the pre-determined portion of sites requires significant processing and resources.
In addition, it has been found that a problem associated with the use of silver-exchanged strong acid cation exchange resins is that the silver may actually be displaced by corrosion metals, as described in U.S. Pat. No. 5,344,976. The patent describes the use of a cation exchanger in the acid form to remove at least a portion of the metal ion contaminants such as iron, potassium, calcium, magnesium, and sodium from a carboxylic acid stream prior to contacting the stream with the exchanged strong acid cation exchange resin to remove C1 to C10 alkyl iodide compounds, hydrogen iodide or iodide salts. However, this process does not describe purification for low-energy and low-water carbonylation processes as described above that may contain lithium and larger alkyl iodide compounds, in addition to iodides.
In addition, other schemes introduce other contaminants that may need to be removed from the product. For example, it has been well known in the art for some time that adding an alkali component such as KOH to the drying column of a carbonylation purification process is useful to inhibit the buildup of HI in the column. See, e.g., US 2013/0264186 and earlier references. However, this addition introduces a potassium cation into the process that can also displace the silver in a silver-exchanged strong acid cation exchange resin.
Other processes remove corrosion metal contaminants at different stages of the process, for example from the reactant composition. U.S. Pat. No. 4,894,477 describes a process that uses strongly acidic ion exchange resins in the lithium form to remove corrosion metal contaminants. U.S. Pat. No. 5,731,252 describes contacting the catalyst solution with an ion exchange resin bed, in the lithium form, while requiring simultaneous addition of a sufficient amount of water to allow the corrosion metal salts in the catalyst medium to dissociate so that ion exchange can occur and the corrosion metals can be removed from the reactor catalyst solution.
While the above-described processes have been successful, the need exists for process for improved processes for producing acetic acid, in particular, low water and low energy processes and methods for removing contaminants from those processes.